1. Field of the Invention
The present invention relates to a discharge head for discharging liquid such as liquid medicine and turning the liquid into liquid droplets, and also to a droplet discharging device.
2. Related Background Art
A method of administering a drug to a patient by using an inhaler and causing the patient to inhale very fine droplets of the drug (liquid medicine) formed by dispersing the drug into a solution is known. Such an inhaler includes a reservoir for storing liquid medicine, a discharge head for discharging the liquid medicine and a control section for controlling the operation of each of the components. Particularly, for a patient to inhale liquid medicine by way of the alveoli of the lung, the droplets should be made to have a diameter not greater than 10 μm so as to be able to reach the alveoli. Therefore, the inhaler is required to be able to produce a large number of droplets having such a small droplet diameter.
Many discharge methods have been proposed to date, and inhalers using such methods have been developed and put to practical use. Popular discharging methods include one for vibrating liquid medicine by means of ultrasonic waves, producing droplets from the surface of the liquid medicine and atomizing them by vibrations, in order to allow the patient to inhale the drug. A technique of applying a voltage to a piezoelectric element to oscillate the element and generate ultrasonic waves has been and is being popularly employed. A filtering method of allowing only small droplets to pass through a mesh that is a thin film having a plurality of very small holes in order to improve the distribution of diameters of atomized droplets has also been proposed.
Other known discharge methods include one for arranging a heater/heating mechanism near the discharge ports of an inhaler, energizing the heater to boil liquid and discharging droplets from the discharge ports (see International Publication WO 95/01137). A discharge head using this method includes a heater board base member where a liquid-chamber/flow-path forming member and heater wiring are arranged and an orifice plate having discharge ports and bonded to the base member. While this method allows the inhaler to be made smaller and allows droplets to be micronized with ease, it is accompanied by problems including that a large amount power needs to be supplied to the heater to increase the discharge rate, making it difficult in practice to increase the discharge rate and also meaning that the drug can be scorched by heat. Liquid medicine may be discharged from discharge ports by applying pressure to the liquid medicine by way of displacement of piezoelectric crystals so as to push out droplets from the discharge ports as disclosed in WO 95/01137.
Still other known discharge methods include one for guiding liquid medicine under high pressure to the discharge ports of a discharge head and spraying droplets from the discharge ports at high speed (pressure exertion method). With this method, the liquid medicine passes through fine discharge ports and a liquid jet that is a continuous fast flow of liquid is discharged from each of the discharge ports. Particularly, when the diameter of the discharge ports is small, the liquid jets are broken into droplets by the wave that is autonomously generated along the lateral surface of the liquid jets, thus producing droplets when the liquid jets proceed a certain distance from the respective discharge ports. The distance from a discharge port that is required for the liquid jet to be broken up in this way is referred to as the droplet forming length. The droplet forming length can vary depending on the velocity of the liquid jet, the diameter of the discharge ports, the surface tension of the liquid medicine and the viscosity of the liquid medicine. The liquid medicine is discharged as liquid jets from the discharge ports as far as the droplet forming length but then proceed beyond the droplet forming length in the form of droplets. The pressure exertion method provides advantages including that it is free from the problem of scorching the liquid medicine that accompanies the method of WO 95/01137, the power required to discharge the liquid medicine can be reduced, and a high discharge rate can be achieved with ease.
Meanwhile, with the pressure exertion method, the meniscus pressure that is produced at the discharge ports due to the surface tension of the liquid medicine increases as the diameter of the discharge ports is reduced, consequently raising the discharge pressure necessary for discharging the liquid medicine as liquid jets. Since a liquid jet is produced when the inertial force of the liquid jet exceeds the surface tension, the condition to be met for a liquid jet to be discharged can be determined by the Weber number defined by formula (1) below:ρ×D×V2/σ,  (1)where ρ is the liquid density, D is the diameter of the discharge port, V is the velocity of the liquid jet at the discharge port surface and σ is the surface tension of the liquid. A liquid jet is discharged when the value given by formula (1) exceeds a certain level. The velocity of the liquid jet at this level is defined as the liquid jet forming velocity Vd. If the discharge pressure in the discharge head that corresponds to the liquid jet forming velocity Vd is Pd, the mathematical expression for Pd has a term that is proportional to the square of Vd and, approximately, since discharge of the liquid jet is achieved with a constant value of the expression in formula (1), the discharge pressure increases as the nozzle diameter is reduced.
Droplets of a size on the order of several microns need to be produced for a discharge head to be effectively used in an inhaler. When droplets of this size are discharged from a discharge head with a pressure exertion method, the discharge pressure will be not less than 2 MPa, as described in International Publication WO 94/27653. Therefore, inhalers adopting the pressure exertion method are required to have a structure that can withstand high liquid pressure at the part thereof that is brought into contact with the liquid medicine, and also a mechanism for exerting high pressure, and thus they face a problem of durability and become more difficult to make smaller. This is illustrated in another technical field, that of printers, since printers of a continuous ink-jet type adopt a pressure exertion method and employ a pump as the pressure exerting mechanism, so that printers are inevitably large.
WO 94/27653 discloses an inhaler adopting such a pressure exertion method. According to the patent document, a discharge head and a reservoir are combined to form an integral structure, which is a cartridge, and the material of the cartridge displays plasticity. When the inhaler is administering a drug, a piston is pushed out by spring force to partly crush the cartridge and produce a large pressure that discharges liquid medicine from the discharge ports. With this method, the cartridge is disposable, and is replaced each time after administering the drug. This method is referred to as a single-dose type, whereas a method of administering a drug several times without replacing the cartridge or the head is referred to as a multi-dose type.
A single-dose inhaler as disclosed in WO 94/27653 requires the cartridge to be replaced each time after drug administration, which makes operation of the inhaler cumbersome and liable to induce an accident. Additionally, the cost of the cartridge material rises. Therefore, it is desirable to realize a multi-dose inhaler using the pressure exertion method. For this purpose, the discharge head is required to have a sufficient degree of durability and withstand high liquid pressure even after a plurality of times of drug administration. Additionally, the inhaler needs to be made small, from the viewpoint of portability. For this purpose, the inhaler requires a low discharge pressure, and also a simplified structure for each part of the inhaler. The discharge pressure of an inhaler is the sum of the pressure necessary for energizing droplets to discharge them at the discharge velocity Vd or at a higher velocity and the pressure loss PL at the discharge port. The PL at an ordinary discharge port can approximately be expressed by formula (2) below that conforms to the Hagen-Poiseuille equation:PL=(C×μ×T×V)/D2,  (2)where μ is the viscosity of droplets, T is the thickness of the orifice plate and C is a coefficient of proportion that is determined according to the structure of the orifice plate. From formulas (1) and (2), it will be seen that the discharge pressure can be reduced by modifying the physical properties of the liquid medicine and the design of the orifice plate. The surface tension and the viscosity of the liquid medicine cannot be modified very significantly, due to the desire for maximizing the effect of the drug. As for the orifice plate, the value of PL in formula (2) is reduced by increasing the diameter of the discharge ports, increasing the number of discharge ports and/or reducing the thickness of the orifice plate to enable reduction of the discharge pressure.
Generally, the diameter and the number of the discharge ports cannot normally be modified because they determine the droplet diameter and the discharge rate, respectively. On the other hand, the pressure loss can be reduced at the discharge ports to lower the discharge pressure, by reducing the thickness of the orifice plate. However, the strength of the orifice plate is reduced when the orifice plate is made thin, which makes it difficult to bond the orifice plate and the discharge head. Even if they can be bonded, the thin orifice plate can no longer withstand the high liquid pressure at the time of discharging the liquid medicine to give rise to liquid leakage from the junction and destroy the discharge head. Additionally, the orifice plate itself will be deformed and destroyed under the high liquid pressure.
It is difficult for the conventional design of arranging an orifice plate at part of the container for containing a solution under high water pressure as disclosed in WO 94/27653 to secure durability because the orifice plate is exposed to high water pressure for a long time when the discharge head is operated repeatedly. However, durability does not become a serious problem when the orifice plate is exposed to high water pressure only instantaneously, as in cases where the discharge head is disposable as disclosed in WO 94/27653.
Additionally, the problem that the durability of the discharge head is reduced as the orifice plate is made thinner to reduce the discharge pressure remains unsolved.